Worldwide, most HIV-1 infections in both women and men result from mucosal transmission. In developing countries, the majority of women are infected via the vaginal route of transmission. Although homosexual transmission via anal intercourse among males still accounts for most new infections in developed countries, the incidence of heterosexual transmission in women is rapidly increasing in the U.S. A microbicide that can be applied before intercourse that would consistently and safely prevent infection with HIV-1 could save millions of lives. However, while a microbicide may demonstrate efficacy in preventing HIV-1 infection of cells in vitro, several additional factors may affect its ability to prevent viral transmission across an intact mucosal surface. For example, activated macrophages and dendritic cells are rare in PBMCs, yet they are abundant in mucosal tissues. Since the latter may be the major initial target cell for viral transmission across the mucosal surface, it is critical to examine the efficacy of candidate compounds in preventing viral transmission across an intact mucosal surface using a relevant animal model. Finally, the long-term effects of the microbicide on the vaginal or rectal mucosa must be carefully examined, since repeated use may induce local inflammation, and therefore, increased rates of viral transmission. The experiments in this proposal are thus designed to test the efficacy and safety of candidate microbicides in preventing SHIV-1/SIV transmission across the vaginal and rectal mucosa using the highly relevant rhesus macaque model. The Specific Aims of this proposal are: 1- To screen promising candidate compounds (identified by in vitro experiments) for efficacy in the SHIV-162P/SHIV-89.6P macaque vaginal and rectal transmission models. 2- To rigorously examine the efficacy of the most promising compounds identified in Aim 1 in a multiple exposure model that will closely mimic a practical microbicide application in humans. 3- To examine the histopathologic and immunophenotypic changes in the vagina and rectum after single and repeated exposures to the compounds and viral challenge. Combined, these experiments are designed to test microbicides alone, in combination, and in different formulations in the macaque mucosal transmission model, with the ultimate goal of identifying a compound that could be safely and effectively used as a prophylactic microbicide to prevent sexual transmission of HIV-1.